The operation of an unmanned aerial vehicle (or “UAV”) is dependent upon a combination of four forces, namely, thrust, drag, weight and lift, the net effects of which may determine an extent and a direction of a velocity of the aerial vehicle. Thrust is a force that is typically generated by one or more aerial propulsors or propulsion units such as rotating bladed propellers or jet engines. Thrust may have a magnitude defined by one or more operating characteristics of the propulsor, e.g., a rotating speed, a number of blades, or sizes of blades of a propeller, or an amount or speed of exhaust expelled from a jet engine, as well as a direction defined by an orientation of the propulsor with respect to an airframe of an aerial vehicle. Thrust is necessary in order to overcome drag, which is a resistive aerodynamic force that is directed in an opposite direction to a direction of travel of the aerial vehicle, due to air that opposes the forward motion of the aerial vehicle. Weight is a force resulting from the Earth's gravitational pull acting on a center of mass (or center of gravity) of the aerial vehicle, in a vertical direction toward the Earth's center. Lift is another aerodynamic force that is generated by propellers, or from flows of air over wings or other control surfaces. Lift counteracts the effects of weight on an aerial vehicle, at least in part. Thrust, drag, weight and lift acting on an aerial vehicle must be placed in balance in order to ensure that the aerial vehicle operates at a desired and safe velocity.
With the exception of weight, each of the forces acting on an operating unmanned aerial vehicle may be affected by wind passing above, below or around the aerial vehicle. Wind may include a number of components that impact an amount of lift generated by a fixed or rotating wing on an aerial vehicle, as well as an extent of thrust or drag applied to the aerial vehicle. For example, a headwind is wind blowing on a front of an aerial vehicle, opposite to its direction of travel, while a tailwind is wind that blows from behind an aerial vehicle, in its direction of travel. Meanwhile, a crosswind is wind that blows laterally into an aerial vehicle, parallel to ground below the aerial vehicle and perpendicular to its direction of travel. Updrafts and downdrafts are winds that blow perpendicular to the ground and originate above or below an aerial vehicle, respectively. Wind that contacts an aerial vehicle typically includes one or more components (e.g., headwinds, tailwinds, crosswinds, updrafts or downdrafts) that impart forces on the aerial vehicle from a number of different directions.
Today, unmanned aerial vehicles are being utilized in an ever-increasing number of missions, including but not limited to surveillance, monitoring or delivery operations. The use of an unmanned aerial vehicle, as opposed to a manned aerial vehicle, carries a number of advantages deriving from the fact that such vehicles are not required to carry humans. For example, unmanned aerial vehicles are typically rigid structures that are lighter, smaller and less expensive than their manned counterparts, and may be used in missions for which human safety or the costs or risks of human operation may be prohibitive. Unfortunately, however, the rigid construction of unmanned aerial vehicles, and their inherent lack of human onboard control, requires unmanned aerial vehicles to adapt to changing circumstances, including planned or unplanned variations in environmental conditions or operational requirements, or material or component failures.
The capacity to rapidly adapt to changing circumstances is particularly acute when aerial vehicles are operating in or transitioning to a hovering flight mode, as a balance between thrust, drag, weight and lift forces acting on an aerial vehicle that is traveling at low speeds or is hovering may be easily upset.